Initiated by the discovery of topological insulators (TIs) whose surface or edge hosts gapless states despite the insulating nature of bulk [1]}, [2]}, [3]}, the search for new quantum states of matter characterized by the non-trivial topology is becoming one of major challenges in condensed-matter physics. While TI and topological superconductor (TSC) - a superconducting analog of TI - commonly contain a finite energy gap in the bulk (band gap and superconducting gap, respectively), the classification of band topology has been successfully extended to the systems with gapless bulk excitations, leading to the discovery of topological semimetals (TSMs). The first concrete example of TSMs is the three-dimensional (3D) topological Dirac semimetal such as Na\(_3\) Bi and Cd\(_3\) As\(_2\)  [4]}, [5]}, [6]}, [7]}, [8]} where a linearly dispersive bulk valence band (VB) and a conduction band (CB) contact each other at a discrete point in the 3D k space. Weyl semimetal is also characterized by the point touching of VB and CB, while the bulk band degeneracy is lifted by the breaking of time-reversal or space inversion symmetry [9]}, [10]}, [11]}, [12]}, [13]}. There is another type of TSM called line-nodal semimetal where the crossing point extends one-dimensionally in the k space (nodal line or loop), being protected by the crystal symmetry such as mirror reflection or nonsymmorphic glide mirror symmetry [14]}, [15]}, [16]}, [17]}, [18]}, [19]}, [20]}, [21]}, [22]}, [23]}, [24]}, [25]}, [26]}, [27]}, [28]}. While such TSMs, or more widely, materials containing Dirac/Weyl or line nodes in the bulk-band structure, are already a useful platform to realize exotic quantum phenomena such as unusual magneto-transport properties, the coupling of nodal electrons to superconductivity is another intriguing target of intensive investigations because such a coupling would provoke the topological superconductivity [29]}. However, this essential issue remains highly unexplored, because superconducting materials whose low-energy excitations are characterized by Dirac/Weyl or line-nodal electrons are generally rare [16]}, [31]}, [32]}, [33]}.
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